Present day power tongs that are employed for coupling and decoupling threaded pipe sections are typically subject to one or more of a number of practical problems. Some examples are found in systems for the engagement and disengagement of sections of a casing or pipe string that is to be lowered into or removed from a well bore. Extremely high torques may have to be applied, due to combinations of factors such as the presence of corrosion, the existence of distortion, and pipe size and weight. High shock forces arise, both in the "make" direction of rotation when a shoulder is suddenly encountered, and in the "break" direction at initial disengagement. Moreover, the forces and pressures involved are at such levels that operation is seldom smooth and uniform. For example, in excess of 50,000 foot-pounds of torque may be exerted, and the power tongs may be required to withstand a spreading force of in excess of 200,000 pounds, while relatively small die elements engage the pipe with extremely high force loadings. Consequently it is common for slippage to occur, for the pipe surfaces to become galled, and sometimes for the pipe to be broken.
There is a profusion of patent art in this field, all of which evidences the extent and variety of the problems that are encountered and the numerous attempts that have been made to overcome such problems. By way of reference, the following patents are illustrative of the state of the art: U.S. Pat. Nos. 3,875,826, Dreyfuss et al, Apr. 8, 1975; 2,950,630, J. C. Mason, Aug. 30, 1960; 2,879,680, A. W. Beeman et al, Mar. 31, 1959; 3,147,652, George, Sept. 8, 1964; 3,140,624, George, July 14, 1964; 3,023,651, Wallace, Mar. 6, 1962; 2,846,909, Mason, Aug. 12, 1958; 3,064,413, Mason, Apr. 23, 1963; 2,933,961, Adams, Apr. 26, 1960; 3,691,875, Geczy et al, Sept. 19, 1972; 3,704,639, Foss, Dec. 5, 1972; 2,550,045, De Hetre, Apr. 24, 1951; 3,793,913, Wilms, Feb. 26, 1974; 3,996,320, Brown, Dec. 4, 1973; 3,706,243, Wilms, Dec. 19, 1972; 3,180,186, Catlind, Apr. 27, 1965; 3,722,603, Brown, Mar. 27, 1973; 3,635,105, Dickmann et al, Jan. 18, 1972; 3,371,562, Kelley, Mar. 5, 1968; 3,507,174, Dickmann, Apr. 21, 1970.
In order to react against the spreading forces that are created, some systems of the prior art utilize a full rotary construction. That is, the principal operative element is a closed ring rotary which is strong enough to withstand the spreading forces but which because of its closed ring design can only be installed or removed over the end of a pipe. Other workers in the art, such as evidenced by Dreyfuss et al U.S. Pat. No. 3,875,826, have devised specialized ring configurations in which annular elements open on one side may be rotated relative to each other so as to provide a form of equivalent to a closed-ring system. In this system, the heads carrying the gripping dies that engage the pipe are mounted to slide in the rotary. Problems that arose during testing were such that kept the design from being commercially marketed. In practical systems it was found that the Dreyfuss design was such that unbalanced forces were exerted on the dies and the ring system, causing uneven gripping, and the opening in the ring system caused undue wear in the drive gear system. Because of these and other factors the commercial version of this system encountered substantial operative difficulties and is understood to have been withdrawn from the market. Another example of power tongs which can be opened for side access to a pipe is provided by Wallace U.S. Pat. No. 3,023,651, in which a number of hinged sections are employed in a massive and heavy structure that is difficult to place in position and operate, and is not amenable to use with different sizes of pipe.
A different approach is evidenced by Eckel U.S. Pat. Nos. 4,084,453 and 4,089,240. In both of these constructions a partial ring having internal cam surfaces is disposed within a gear driving mechanism that substantially surrounds the partial ring. In the former patent the dies that engage the pipe are mounted on pivotable link members, and a critical "cam angle" is defined that must be between 1/2 and 51/2.degree.. This angle is additionally critical because the two dies on a link member are at different radii relative to the pivot axis, and act symmetrically on a given size pipe only within a very small part of their travel. In the latter patent each member, holding the dies provides rectilinear movement, but this is achieved by employing a guide rod sliding within a guide passage in the member. The arrangement is relatively complex and imparts substantial bending moments on the guide rod and the receiving bearings. A drag mechanism forms an essential part of the system, in order to provide relative movement between the partial ring rotary and the die carrier.
The examples of prior art constructions mentioned also are susceptible to one or more of a variety of other problems. For example, fragments and dirt can enter into the cam devices that are typically used to urge the dies into engagement with the pipe, damaging the cams and causing the dies to lock in or out of position.
Many designs also are such that die loading becomes increasingly asymmetrical as pipe size is reduced, substantially increasing die wear and the probability of damage. A power tong should preferably be able to cover a range of pipe sizes without difficulty, and if a further pipe size change is needed it should be effected with only an interchange of parts. Maintenance and life problems have an economic significance far in excess of the cost of the dies or even the pipe involved, because the down time that results when replacements or repair must be made involves not only material costs but also drilling crew costs and the continuing charges for other specialized tool equipment present at the drill rig. Thus a power tong system which requires frequent replacement of dies or other elements or which causes undue damage to sections in a pipe string would be far inferior to a power tong system which operates steadily and uniformly.
As those skilled in the art are aware, the extremely high stresses and abrupt shocks encountered in operation are usually attended by visible strains on the equipment and by vibrations and sharp impacts which results in a very short fatigue life for the parts involved and the unit as a whole. These are caused by overload or unbalanced force conditions which are further evidenced by undue wear, slippage or equipment damage.
The referenced patents, and the pertinent literature, place substantial emphasis on the use of drag or braking techniques to secure proper biting of the dies relative to the pipe. As the rotary is driven the head or other member supporting the dies is frictionally restrained to insure that the dies do not simply rotate with the rotary. Inasmuch as most power tong systems use approximately 2500 to 3000 psi of hydraulic pressure for driving purposes, and inasmuch as the drag typically requires a pressure of approximately 700 psi, a substantial part of the available energy is effectively used only for overcoming this braking friction. The driving energy available for the power tongs thus is reduced in at least the same proportion. Actually, the total loss in efficiency can be substantially greater, because both the hydraulic system and the prime mover may be operating in less efficient regimes. That is, at higher pressure levels torque may not increase in linear fashion.